1. Field of the Invention
The present invention relates to a method for carrying out an extrusion, wherein an extrusion material is melted and delivered under a pressure by an extruder and then formed into a desired shape by being passed through a die. Further, the present invention relates to an apparatus for carrying out the above method.
2. Description of the Related Art
An extrusion apparatus generally has an extruder, a die, a cooling apparatus, and so on, arranged in series, and an extrusion material such as a plastic material is melted by the extruder and delivered therefrom under a pressure. The delivered material is then introduced into the die, and passed through the die to impart a desired shape to the material. The material extruded from the die is then cooled by the cooling apparatus.
Currently, extrusion materials such as a plastic material have an ever-increasing molecular weight, or contain an increasingly higher content of a filler such as inorganic material, to improve the physical properties of the products, such as strength, rigidity, slidability, or the like.
The plastic material improved as above, however, exhibits poor fluidity in the die during extrusion, and as a result, the surface of a formed article becomes rough (melt fractures), or a pressure in the die becomes extremely high, and the die is deformed to thereby cause distortion of the formed article. To avoid the above defects, it is necessary in the conventional apparatus to lower the extrusion speed, i.e., reduce the producing speed.
Under the above circumstances, attempts have been made to remedy such disadvantages by improving the fluidity of the extrusion material in the die during extrusion.
For example, Japanese Unexamined Patent Publication No. 57-51441 proposes a process for increasing the extrusion speed by flowing the material while applying a mechanical vibration to the die, to thereby improve the fluidity of the extrusion material. This process is not satisfactory, however, in that the mechanical vibration is merely applied to the fixed die, and thus, in practice little vibration is transmitted to the die, and therefore, satisfactory results cannot be obtained.